Formation of Fractures Within Horizontal Well

ABSTRACT

Producing transverse fractures in a horizontal well may be achieved at a relatively lower fracturing pressure by forming one or more tunnels extending from the horizontal wellbore. One or more tunnels may be formed at each location along the horizontal wellbore where a transverse fracture is desired. The tunnel(s) may be formed mechanically, optically, or hydraulically. Further, fracturing may be formed at a lower pressure than would otherwise be required to form transverse fractures from a horizontal wellbore. According to some implementations, the transverse fractures may be formed without isolating a portion of the horizontal wellbore.

TECHNICAL FIELD

This disclosure relates to forming transverse fractures into asubterranean zone from a horizontal well and more particularly to usinga tunnel extending from the horizontal wellbore as a transverse fractureinitiation location.

BACKGROUND

Reservoir stimulation may be used to enhance recovery of reservoirfluids from a subterranean reservoir or zone. An example reservoirstimulation is hydraulic fracturing (interchangeably referred to as“fracturing”) in which fluid is pumped into a wellbore at an elevatedpressure to form one or more fractures in the subterranean reservoirbordering the wellbore. The fractures formed during fracturing provideflow conduits emanating from the wellbore, providing flowpaths for thereservoir fluid to collect in the wellbore and subsequently be producedto the surface.

SUMMARY

One aspect of the present disclosure is directed to a method of formingtransverse fractures extending from a horizontal wellbore. The methodmay include forming a wellbore having a horizontal wellbore portionwithin a subterranean zone and forming a tunnel extending from thehorizontal wellbore portion into the subterranean zone towards theoverburden. The tunnel may be formed with a length adapted to initiate afracturing extending from the tunnel along a longitudinal axis thereofbeing influenced insignificantly by the horizontal wellbore portion. Themethod may also include applying fluid pressure to an interior of thehorizontal wellbore portion at a location proximate the tunnel to form afracture extending from the tunnel along a longitudinal axis thereof andpropagating the initiated fracture to encompass the horizontal wellboreportion.

A second aspect is directed to a wellbore system including a horizontalwellbore extending through a subterranean zone and at least one tunnelextending from the horizontal wellbore into the subterranean zonetowards the overburden. The at least one tunnel may have a lengthadapted to form transverse fractures relative to the horizontalwellbore.

A third aspect is directed to a method of forming fractures transverseto a horizontal wellbore including forming a wellbore having ahorizontal wellbore portion within a subterranean zone and forming atunnel extending from the horizontal wellbore portion into thesubterranean zone towards the overburden. The tunnel may be formed witha length such that the horizontal wellbore portion has insignificanteffects on formation of a fracture extending from the tunnel along alongitudinal axis thereof The method may also include applying fluidpressure to an interior of the horizontal wellbore portion at a locationproximate the tunnel to form the fracture extending from the tunnelalong the longitudinal axis thereof and propagating the initiatedfracture to encompass the horizontal wellbore portion.

One or more of the aspects may include one or more of the followingfeatures. Forming a tunnel extending from the horizontal wellboreportion into the subterranean zone towards the overburden may includeinserting a tool in the horizontal wellbore portion and orienting thetool into a desired orientation to form the tunnel. Forming a tunnelextending from the horizontal wellbore portion into the subterraneanzone towards the overburden may include forming a first tunnel extendingfrom a first portion of the horizontal wellbore portion and forming asecond tunnel extending from a second portion of the horizontal wellboreportion opposite the first portion. Forming a tunnel extending from thehorizontal wellbore portion into the subterranean zone towards theoverburden may include forming the tunnel with one of a hydrajet, alaser, or a drilling tool. Forming the tunnel with a hydrajet mayinclude disposing a hydrajet into the horizontal wellbore portion at adesired location therein, orienting the hydrajet to form the tunnel, andoperating the hydrajet to impinge a fluid flow onto a surface of thehorizontal wellbore portion to form the tunnel. Forming the tunnel witha laser may include disposing a laser into the substantially horizontalwellbore portion, orienting the laser to form the tunnel, and operatingthe laser to form the tunnel. Forming the tunnel with a drilling toolmay include disposing a drilling tool into the substantially horizontalwellbore portion, orienting the drilling tool to form the tunnel, andoperating the drilling tool to form the tunnel.

One or more of the aspects may also include one or more of the followingfeatures. Forming a tunnel extending from the horizontal wellboreportion into the subterranean zone towards the overburden may includeforming a tunnel extending from the horizontal wellbore portion into thesubterranean zone towards the overburden at two or more differentlocations along an axial length of the horizontal wellbore portion. Aportion of the horizontal wellbore may be isolated at a location of thetunnel before applying the fluid pressure. Forming a tunnel extendingfrom the horizontal wellbore portion into the subterranean zone towardsthe overburden may include forming the tunnel with a length of at leastone and a half (1.5) times a radius of the horizontal wellbore portion.Forming a tunnel extending from the horizontal wellbore portion into thesubterranean zone towards the overburden may include forming the tunnelwith a length of at least three (3) times a radius of the horizontalwellbore portion. Forming a tunnel extending from the horizontalwellbore portion into the subterranean zone towards the overburden mayinclude forming the tunnel with a length of at least six (6) times aradius of the horizontal wellbore portion.

One or more of the aspects may additionally include one or more of thefollowing features. At least a portion of the horizontal wellbore mayinclude a slanted portion, and the tunnel may extend from the slantedportion of the horizontal wellbore. The at least one tunnel extendingfrom the horizontal wellbore into the subterranean zone towards theoverburden may include a first substantially vertical tunnel extendingfrom a first portion of the horizontal wellbore and a secondsubstantially vertical tunnel extending from a second portion of thehorizontal wellbore along a perimeter thereof opposite the firstportion. The at least one tunnel having a length adapted to formtransverse fractures relative to the horizontal wellbore may include atunnel having a length of at least one and a half (1.5) times a radiusof the horizontal wellbore, a tunnel having a length of at least three(3) times a radius of the horizontal wellbore, or a tunnel having alength of at least six (6) times a radius of the horizontal wellbore.

One or more of the aspects may further include one or more of thefollowing features. Forming a tunnel extending from the horizontalwellbore portion into the subterranean zone towards the overburden mayinclude forming the tunnel with a length of at least one and a half(1.5) times a radius of the horizontal wellbore portion, forming thetunnel with a length of at least three (3) times a radius of thehorizontal wellbore portion, or forming the tunnel with a length of atleast six (6) times a radius of the horizontal wellbore portion. Forminga tunnel extending from the horizontal wellbore portion into thesubterranean zone towards the overburden may include inserting a tool inthe horizontal wellbore portion and orienting the tool into a desiredorientation to form the tunnel. Forming a tunnel extending from thehorizontal wellbore portion into the subterranean zone towards theoverburden may include forming the tunnel with one of a hydrajet, alaser, or a drilling tool. Forming a tunnel extending from thehorizontal wellbore portion into the subterranean zone towards theoverburden may include forming a tunnel at two or more differentlocations along an axial length of the horizontal wellbore portion. Aportion of the horizontal wellbore portion may be isolated at a locationof the tunnel before applying the fluid pressure.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features,objects, and advantages will be apparent from the description anddrawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a wellbore extending from a terranean surface into asubterranean zone.

FIG. 2 shows a longitudinal fracture extending from a horizontal portionof a wellbore.

FIG. 3 shows a longitudinal fracture extending from a horizontalwellbore.

FIG. 4 shows transverse fractures extending from a horizontal portion ofa wellbore.

FIG. 5 shows a transverse fracture extending from a horizontal wellbore.

FIG. 6 is a view along an axis of a horizontal wellbore in which atransverse fracture extends from the horizontal wellbore.

FIG. 7 shows a longitudinal fracture extending from a vertical wellbore.

FIG. 8 shows a tunnel extending from a portion of a horizontal wellbore.

FIG. 9 shows first and second tunnels extending vertically from oppositelocations along a perimeter of a horizontal wellbore.

FIG. 10 is a cross-sectional view along A-A in FIG. 8 showing atransverse fracture surrounding the horizontal wellbore that wasinitiated from the tunnel.

FIG. 11 is a cross-sectional view along B-B in FIG. 9 showing atransverse fracture surrounding the horizontal wellbore that wasinitiated from the tunnel.

FIGS. 12-14 illustrate the formation of a transverse fracture relativeto a horizontal wellbore.

FIG. 15 is a summary table of experimentation data.

FIGS. 16-22 are schematic diagrams illustrating the configuration of thebores extending through various test samples.

DETAILED DESCRIPTION

Producing transverse fractures in a horizontal well is described. FIG. 1shows a wellbore 10 having a substantially horizontal portion(hereinafter referred to as “horizontal wellbore”) 20. The wellbore 10extends from a terranean surface 30 and extends into a subterranean zone40. During the producing life of the wellbore 10, such as afterformation of the wellbore 10 or at one or more occasions after thewellbore 10 has been producing reservoir fluids, the subterranean zone40 may be subjected to a fracturing operation to enhance production ofthe reservoir fluids.

Previously, fracturing was performed, for example, by isolating arelatively small section of the wellbore 10 (such as with one or morepackers) and injecting a fluid into the isolated section at highpressure. The high pressure fluid increased the stress state of thesubterranean zone 40 resulting in the formation of fractures extendinginto the subterranean zone. However, controlling the orientation of theproduced fracturing with respect to the wellbore 10 using thisfracturing method was difficult, resulting in high friction pressure andsometimes creating axial fractures (also referred to herein aslongitudinal fractures). In some instances, as the axially fracturespropagated, the axial fractures would become re-oriented so as to beperpendicular to the minimum stress of the subterranean zone 40. There-orientation of these fractures may lead to a sand out. That is, thefracture is unable to accept additional proppant during the fracturingoperation and only the carrier fluid is injected into the formationthrough these fractures. FIGS. 2 and 3 illustrate longitudinal fractures50 extending longitudinally along an axis 60 of the horizontal wellbore20. Longitudinal fractures, though, are not optimum and generally resultin reduced production in comparison to transverse fractures formed in ahorizontal wellbore. FIGS. 4-6 illustrate transverse fractures 70 formedin the subterranean zone 40 bordering the horizontal wellbore 20.Further, longitudinal fractures are generally more likely to result whenfracturing a horizontal wellbore.

Longitudinal fractures are also more likely to be formed in verticalwellbores at lower fluid pressures. That is, longitudinal fractures areformed from a vertical wellbore at a lower breakdown pressure. FIG. 7illustrates longitudinal fractures 50 extending from a vertical well.This characteristic can be utilized to promote formation of transversefractures in a horizontal well. Particularly, one or more bores ortunnels 80 may be extended from a horizontal wellbore 20 and used topromote the formation of a transverse fracture about the horizontalwellbore 20. The one or more tunnels 80 may extend towards theoverburden. Generally, this means that the one or more tunnels 80 extendvertically or substantially vertically from the horizontal wellbore 20.For the purposes of this disclosure, forming the one or more tunnels 80towards the overburden is described as being formed vertical orsubstantially vertical. However, it is understood that the tunnels 80may be formed in a direction other than vertical or substantiallyvertical in situations where the overburden is not at a locationvertically offset from the horizontal wellbore 20. Further, the one ormore tunnels 80 may deviate from vertical or substantial vertical by15°. The tunnels 80 promote the initiation and propagation of fracturesthat are independent of influences associated with horizontal andvertical orientations aspects of the well.

FIG. 8 shows a single tunnel 80 extending substantially vertically froma first portion 82 of the horizontal wellbore 20, while FIG. 9 shows apair of tunnels 80 extending from the horizontal wellbore 20. In FIG. 9,one of the tunnels 80 extends from the first portion 82 of thehorizontal wellbore 20, and the second tunnel 80 extends from a secondportion 84 of the horizontal wellbore 20, opposite the first portion 82.Further, both tunnels 80 are oriented vertically or substantiallyvertically so as to promote the formation of the transverse fracturerelative to the horizontal wellbore 20.

The horizontal wellbore 20 may also include numerous tunnels 80 formedalong the length of the horizontal wellbore 20. Particularly, a tunnel80 may be included on the horizontal wellbore 20 at any location where atransverse fracture is desired. Thus, the number of tunnels 80 formedinto the subterranean zone 40 from the horizontal wellbore 20 may bedependent upon the number of transverse fractures 70 desired.Consequently, the number of tunnels may be determined according to thedesign of the stimulation activity.

FIGS. 10 and 11 show cross-sectional views of the horizontal wellbore 20along lines A-A and B-B, respectively. FIGS. 10 and 11 show exampletransverse fractures 70 extending into the subterranean zone 40 thatwere initiated at the tunnels 80.

The tunnels 80 may be formed in any number of different ways. Forexample, one or more of the tunnels 80 may be formed mechanically, suchas by drilling into the reservoir from the horizontal wellbore 20.According to other implementations, one or more of the tunnels 80 may beformed using one or more lasers. A laser device may be included on atubing string extending into the horizontal wellbore 20 and used to formthe tunnels 80 therefrom. According to still other implementations, oneor more of the tunnels 80 may be formed with a stream of pressurizedfluid, e.g., by hydrajetting, which forces a concentrated jet of fluidat elevated pressures towards a point within a wellbore. Examplehydrajets that may be used are described in U.S. Pat. No. 5,361,856 andU.S. Pat. No. 5,494,103, each of which is incorporated herein byreference in their entirety. A pressurized fluid is then introduced intothe horizontal well 20 to form the transverse fracture 70.

Unlike perforations formed in a wellbore, the tunnel 80 has a betterdefined elongated shape with less damage to the surrounding subterraneanzone 40. This damage provides leak-off paths for the fracturing fluid toflow off into the subterranean zone 40, thereby reducing the effectivepressure exerted on the subterranean zone 40 to form the fracturestherein, i.e., the damage to the surrounding subterranean zone 40 maycause an increase in the breakdown pressure required to fracture thesubterranean zone 40. Further, during a perforating operation, aplurality of perforations are formed in the subterranean zone 40. Thesemultiple perforations also act to lessen the effect of the pressurizedfluid, because the multiple perforations require more pressure and fluidflow.

Additionally, perforating a wellbore with a hydrajet expels a pluralityof fluid streams through respective nozzles. The fluid streams form aplurality of openings into the subterranean formation from the wellbore.However, the effect of using the plurality of fluid streams results inenlarging the openings into an enlarged cavity formed in thesubterranean zone surrounding the wellbore. Thus, when the pressurizedfluid is introduced into the wellbore for fracturing, the enlargedcavity reduces the effectiveness of concentrating the pressurized fluidto initiate and propagate a fracture in a controlled manner. Further,present hydrajets for perforating a subterranean zone are also deficientin that the nozzles expelling the fluid streams are not capable of beingaligned with a particular orientation within the wellbore and are, thus,incapable of aligning openings formed by the hydrajet with a desiredorientation.

Once the one or more tunnels 80 are formed, the subterranean zone 40 maythen be fractured. According to some implementations, the pressurizedfluid may be introduced into the horizontal wellbore 20 via aconcentrated stream at or near the location of the tunnel(s) 80.Alternately, a portion of the horizontal wellbore 20 including thetunnel(s) 80 is isolated according to any desired manner, and thepressurized fluid is introduced into the isolated portion of thehorizontal wellbore 20 to form the transverse fracture 70.

It is believed that the introduced pressurized fluid works on the tunnel80 to form a longitudinal fracture extending therefrom. As thislongitudinal fracture extends, the fracture encompasses the horizontalwellbore 20, resulting in a transverse fracture with respect to thehorizontal wellbore 20. FIGS. 12-14 illustrate the progression of thefracture believed to occur at a location along a horizontal wellbore 20having a tunnel 80. In FIG. 12, the pressurized fluid (represented bythe plurality of arrows 90) is introduced into the horizontal wellbore20. In FIG. 13, the longitudinal fracture 50 is formed extending fromthe tunnel 80. The initiated longitudinal fracture 50 extends andexpands to encompass the horizontal wellbore 20, thereby resulting in atransverse fracture 70 extending into the subterranean zone 40, as shownin FIG. 14.

The one or more tunnels 80 may have any desired length L. However, asthe length L of the tunnel 80 increases, influences from the horizontalwellbore 20 during fracturing are reduced, resulting in a greaterlikelihood that a transverse fracture with respect to the horizontalwellbore 20 will result. These influences include how the horizontalwellbore 20 affects the stress state of the subterranean zone 40surrounding the tunnels 80 during fracturing. Moreover, for a tunnel 80having a length L of three (3) times the diameter D or six (6) times theradius of the horizontal wellbore, the influences from the horizontalwellbore 20 are negligible. In fact, the influences from the horizontalwellbore 20 are also small with respect to tunnels 80 having lengths Lsmaller than three times the diameter D of the horizontal wellbore 20.For example, a horizontal wellbore 20 may have substantiallyinconsequential effects on a tunnel 80 having a length of three timesthe radius or more (e.g., three, three and a half, four, four and ahalf, five, and five and half times the radius of the horizontalwellbore). A tunnel 80 having a length less than three times the radiusof the horizontal wellbore 20, such as two and a half, two, and even oneand a half times the radius of the horizontal wellbore 20, may also formtransverse fractures notwithstanding the larger, though non-detrimental,effects on the formation of the transverse fractures associated withthese smaller lengths.

A further benefit of using one or more tunnels 80 is that the size ofany isolated portion of the wellbore that may be used can be larger thanconventionally isolated portions. In still other implementations, thepressurized fluid may be introduced into the horizontal wellbore 20 ator near the tunnel(s) 80 without isolating a portion of the horizontalwellbore 20. The manner of injecting the pressurized fluid into thehorizontal wellbore 20 may be selected based on conditions associatedwith the wellbore 10, the subterranean zone 40, and/or any number ofdifferent considerations. For example, porosity of the subterranean zone40, the stress condition of the subterranean zone 40, properties of thereservoir fluids, and/or any other considerations may affect the mannerchosen for introducing the pressurized fluid into the horizontalwellbore 20.

As mentioned above, the tunnel 80 represents a vertical well, and,during fracturing of a vertical well, a longitudinal fracture morereadily forms at a lower pressure. A longitudinal fracture extendingfrom a vertical wellbore more readily occurs because of the stress stateof the subterranean zone. Fractures propagate perpendicular to theminimum principal stress in the subterranean zone. Generally, theminimum principal stress is oriented horizontally. Thus, for a verticalwellbore, longitudinal fractures are more likely to form and form morereadily at lower breakdown pressures. Thus, it is believed that byincluding the tunnel 80 along the horizontal wellbore 20, the tunnel 80acts as a fracture initiation location for a longitudinal fracture withrespect to the tunnel 80. The fracture propagates to the horizontalwellbore perpendicular to the minimum principal stress of thesubterranean zone.

Further, it is believed that the initiated fracture intersects thehorizontal wellbore 20 irrespective of the orientation thereof. That is,the horizontal wellbore 20 may be oriented horizontally or substantiallyhorizontally, or may be slanted within the subterranean zone 40, and thefracture initiated at the tunnel 80 still extends to the horizontalwellbore 20 to form a transverse fracture relative thereto. For example,some horizontal wellbores may be slanted at one or more locations so asto follow a particular formation within a subterranean reservoir. Awellbore extending through a subterranean zone, such as subterraneanzone 40, that is horizontal, substantially horizontal, or that is atleast partially slanted is considered horizontal within the scope ofthis disclosure. Thus, the longitudinal fracture 50 formed from thetunnel 80 represents a transverse fracture with respect to thehorizontal wellbore 20. Consequently, forming the tunnel 80 permits theformation of a transverse fracture along the horizontal wellbore 20using fluid at a lower fluid pressure than would otherwise be requiredto form a transverse fracture along a horizontal wellbore. Use of thetunnel 80 also allows consistent formation of a transverse fracture 70relative to the horizontal wellbore 20. Further, depending on thedownhole conditions, the pressurized fluid may be introduced without theneed for isolating one or more portions of the well. Therefore, use ofthe tunnel 80 has lower associated fracturing costs. Moreover, thetunnel 80 is also believed to essentially eliminate the formation ofmultiple fractures and fracture tortuosity that may result during afracturing operation.

Experimentation, described below, has been performed demonstrating theeffectiveness of a tunnel extending from a horizontal wellbore informing a fracture transverse to the horizontal wellbore at a relativelylow fracturing pressure. FIG. 15 shows test summary data for six testsamples. Each of the test samples were performed by casting a bore and,in some of the experiments, a vertical or substantially vertical tunnelextending therefrom in hydrostone, a gypsum cement. The hydrostone wasprepared having a ratio of 30 parts of water per 100 parts ofhydrostone. FIGS. 16-22 show schematic diagrams of the configuration ofthe bores and, optionally, the tunnels within the hydrostone. Each ofthe test samples were subjected to a 3000 psi pressure on a top surface(as shown in the figures), which resulted in the following stress state:vertical stress=3000 psi, minimum horizontal stress=1800 psi, andmaximum horizontal stress=2500 psi. It is noted that, although some ofthe tests described in FIGS. 15-22 include a wellbore slanted relativeto horizontal (e.g., some wellbores have a slant of 5° relative tohorizontal), the wellbores may have a slant of greater than or less than5° and still be within the scope of the disclosure. For example, in someinstances, the wellbore may have a slant of 15° or greater and a tunnelextending therefrom may still be operable to produce a transversefracture at a relatively low fracture pressure.

FIG. 16 is an elevation view of a schematic of test sample 1. Testsample 1 was formed having a bore 100 having a casing 110. A tunnel 120extends vertically or substantially vertically from the bore 100.(Dimensions of the bore 100 and tunnel 120 are provided in the table ofFIG. 15.) The bore 100 was formed at approximately 5° from horizontal.An interior of the tunnel was in communication with an interior of thebore via an opening formed in the casing 110. As a result of the casing110, fluid pressure introduced into the bore 100 was exerted on thehydrostone (formation 130) via the tunnel 120. As a result, a fractureinitiated at a fluid pressure of 3323 psi transverse to the bore 100.The fracture is believed to have initiated from the tunnel 120 andextended to encompass the bore 100. The fracture extended transverse tothe minimum horizontal stress.

FIGS. 17 and 18 are schematic plan and elevation views, respectively, oftest sample 2. Test sample 2 included an uncased bore 100 formed atapproximately 5° from horizontal. The bore 100 was also formed atapproximately 45° within a horizontal plane, as shown in the plan viewof FIG. 17. The bore 100 of test sample 2 was not cased but did includea tunnel 120 extending vertically or substantially vertically from thebore 100. A fracture transverse to the bore 100 was initiated in thetest sample at 2889 psi. The fracture is believed to have initiated atthe tunnel 120 and extended to encompass the bore 100. The resultingfracture extended past the bore 100 without causing multiple fractures.

FIG. 19 shows a schematic view of test sample 3. Test sample 3 includeda bore 100 that was not cased and did not include a tunnel, and the bore100 was formed at an angle of 5° from horizontal. A fracture extendinglongitudinally along the bore 100 was formed at a fluid pressure of 3903psi introduced into the bore 100.

FIG. 20 shows a schematic elevation view of test sample 4. Test sample 4included an uncased bore 100 formed at an angle of 5° from horizontal. Avertical or substantially vertical tunnel 120 extended from the bore100. Fluid pressure was introduced into the interior of the bore 100 andthe tunnel 120, which caused a fracture transverse to the bore 100 at afluid pressure of 3596 psi. The fracture is believed to have initiatedin the tunnel and propagated to encompass the bore 100.

FIG. 21 shows a schematic elevation view of test sample 5. Test sample 5included an uncased bore 100 formed at an angle of 5° from horizontal.The bore 100 did not include a tunnel extending therefrom. The bore 100was subjected to an internal fluid pressure which, at a fluid pressureof 3525 psi, caused a fracture extending longitudinally along the bore100.

FIG. 22 shows a schematic elevation view of test sample 6, whichincludes a vertical or substantially vertical uncased bore 100. Fluidpressure was introduced into the bore 100, resulting in a fractureextending longitudinally along the bore 100 at a fluid pressure of 2726psi. Test sample 6 illustrates the tendency to forming fracturesextending longitudinally along a vertical bore under stress conditionssimilar to those in an earth formation.

In each of the experiments, the resulting fractures propagatedperpendicular to the minimum stress state. Further, the results showthat, for the bores including vertical or substantially vertical tunnelsextending therefrom, a fracture transverse to the bore was formed at afluid pressure approximately the same as or lower than pressures forminga fracture longitudinal to those bores that did not include a verticalor substantially vertical tunnel extending therefrom.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. A method of forming transverse fracturesextending from a horizontal wellbore comprising: forming a wellborewithin a subterranean zone, the wellbore having a horizontal wellboreportion; forming a tunnel extending from the horizontal wellbore portioninto the subterranean zone towards the overburden, the tunnel formedwith a length adapted to initiate a fracturing extending from the tunnelalong a longitudinal axis thereof being influenced insignificantly bythe horizontal wellbore portion; applying fluid pressure to an interiorof the horizontal wellbore portion at a location proximate the tunnel toform a fracture extending from the tunnel along a longitudinal axisthereof; and propagating the initiated fracture to encompass thehorizontal wellbore portion.
 2. The method of claim 1, wherein forming atunnel extending from the horizontal wellbore portion into thesubterranean zone towards the overburden comprises: inserting a tool inthe horizontal wellbore portion; and orienting the tool into a desiredorientation to form the tunnel.
 3. The method of claim 1, whereinforming a tunnel extending from the horizontal wellbore portion into thesubterranean zone towards the overburden comprises: forming a firsttunnel extending from a first portion of the horizontal wellboreportion; and forming a second tunnel extending from a second portion ofthe horizontal wellbore portion opposite the first portion.
 4. Themethod of claim 1, wherein forming a tunnel extending from thehorizontal wellbore portion into the subterranean zone towards theoverburden comprises forming the tunnel with one of a hydrajet, a laser,or a drilling tool.
 5. The method of claim 4, wherein forming the tunnelwith a hydrajet comprises: disposing a hydrajet into the horizontalwellbore portion at a desired location therein; orienting the hydrajetto form the tunnel; and operating the hydrajet to impinge a fluid flowonto a surface of the horizontal wellbore portion to form the tunnel. 6.The method of claim 4, wherein forming the tunnel with a lasercomprises: disposing a laser into the substantially horizontal wellboreportion; orienting the laser to form the tunnel; and operating the laserto form the tunnel.
 7. The method of claim 4, wherein forming the tunnelwith a drilling tool comprises: disposing a drilling tool into thesubstantially horizontal wellbore portion; orienting the drilling toolto form the tunnel; and operating the drilling tool to form the tunnel.8. The method of claim 1, wherein forming a tunnel extending from thehorizontal wellbore portion into the subterranean zone towards theoverburden comprises forming a tunnel extending from the horizontalwellbore portion into the subterranean zone towards the overburden attwo or more different locations along an axial length of the horizontalwellbore portion.
 9. The method of claim 1 further comprising isolatinga portion of the horizontal wellbore at a location of the tunnel beforeapplying the fluid pressure.
 10. The method of claim 1, wherein forminga tunnel extending from the horizontal wellbore portion into thesubterranean zone towards the overburden comprises forming the tunnelwith a length of at least one and a half (1.5) times a radius of thehorizontal wellbore portion.
 11. The method of claim 1, wherein forminga tunnel extending from the horizontal wellbore portion into thesubterranean zone towards the overburden comprises forming the tunnelwith a length of at least three (3) times a radius of the horizontalwellbore portion.
 12. The method of claim 1, wherein forming a tunnelextending from the horizontal wellbore portion into the subterraneanzone towards the overburden comprises forming the tunnel with a lengthof at least six (6) times a radius of the horizontal wellbore portion.13. A wellbore system comprising: a horizontal wellbore extendingthrough a subterranean zone; and at least one tunnel extending from thehorizontal wellbore into the subterranean zone towards the overburden,the at least one tunnel having a length adapted to form transversefractures relative to the horizontal wellbore.
 14. The wellbore systemof claim 13, wherein at least a portion of the horizontal wellborecomprises a slanted portion and wherein the tunnel extends from theslanted portion of the horizontal wellbore.
 15. The wellbore system ofclaim 13, wherein the at least one tunnel extending from the horizontalwellbore into the subterranean zone towards the overburden comprises: afirst substantially vertical tunnel extending from a first portion ofthe horizontal wellbore; and a second substantially vertical tunnelextending from a second portion of the horizontal wellbore along aperimeter thereof opposite the first portion.
 16. The wellbore system ofclaim 13, wherein the at least one tunnel having a length adapted toform transverse fractures relative to the horizontal wellbore comprisesa tunnel having a length of at least one and a half (1.5) times a radiusof the horizontal wellbore.
 17. The wellbore system of claim 13, whereinthe at least one tunnel having a length adapted to form transversefractures relative to the horizontal wellbore comprises a tunnel havinga length of at least three (3) times a radius of the horizontalwellbore.
 18. The wellbore system of claim 13, wherein the at least onetunnel having a length adapted to form transverse fractures relative tothe horizontal wellbore comprises a tunnel having a length of at leastsix (6) times a radius of the horizontal wellbore.
 19. A method offorming fractures transverse to a horizontal wellbore comprising:forming a wellbore having a horizontal wellbore portion within asubterranean zone; forming a tunnel extending from the horizontalwellbore portion into the subterranean zone towards the overburden, thetunnel formed with a length such that the horizontal wellbore portionhas insignificant effects on formation of a fracture extending from thetunnel along a longitudinal axis thereof; applying fluid pressure to aninterior of the horizontal wellbore portion at a location proximate thetunnel to form the fracture extending from the tunnel along thelongitudinal axis thereof; and propagating the initiated fracture toencompass the horizontal wellbore portion.
 20. The method of claim 19,wherein forming a tunnel extending from the horizontal wellbore portioninto the subterranean zone towards the overburden comprises forming thetunnel with a length of at least one and a half (1.5) times a radius ofthe horizontal wellbore portion.
 21. The method of claim 19, whereinforming a tunnel extending from the horizontal wellbore portion into thesubterranean zone towards the overburden comprises forming the tunnelwith a length of at least three (3) times a radius of the horizontalwellbore portion.
 22. The method of claim 19, wherein forming a tunnelextending from the horizontal wellbore portion into the subterraneanzone towards the overburden comprises forming the tunnel with a lengthof at least six (6) times a radius of the horizontal wellbore portion.23. The method of claim 19, wherein forming a tunnel extending from thehorizontal wellbore portion into the subterranean zone towards theoverburden comprises: inserting a tool in the horizontal wellboreportion; and orienting the tool into a desired orientation to form thetunnel.
 24. The method of claim 19, wherein forming a tunnel extendingfrom the horizontal wellbore portion into the subterranean zone towardsthe overburden comprises forming the tunnel with one of a hydrajet, alaser, or a drilling tool.
 25. The method of claim 19, wherein forming atunnel extending from the horizontal wellbore portion into thesubterranean zone towards the overburden comprising forming a tunnel attwo or more different locations along an axial length of the horizontalwellbore portion.
 26. The method of claim 19 further comprisingisolating a portion of the horizontal wellbore portion at a location ofthe tunnel before applying the fluid pressure.